If this is one of the later sharp roms (I'm not sure about the earlier as I have an SL-C860) then the backup file is actually a tar archive.
You can extract all the files using Linux tar or rename to .tar and extract with WinZip, this will let you do a more granular scan to find the cause.
ClamAV may be of interest (
www.clamav.net) as a freebie AV scanner which runs on Linux.
I think there's actually a port knocking round for the Z but it's an early version.